Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Imagewise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
Thermographic materials of type 1 become photothermographic when a photosensitive agent is present which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Examples of photothermographic materials are the so called "Dry Silver" photographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
GB 1 342 525 discloses a photosensitive heat-processable material comprising a support and contained in a layer or layers thereof: (a) an oxidation/reduction image-forming system comprising: (b) a light-insensitive reducible metal compound and (c) an organic reducing agent therefor, (d) a photosensitive silver compound capable, on exposure to actinic radiation, of forming centres which catalyze the thermally induced oxidation/reduction reaction of (b) with (c) to form a visible image, and (e) a substantially colourless photographic speed-increasing onium halide. From the general formulae given in claim 4 of GB-P 1 342 525, the examples of speed-increasing onium halides given in the specification and the onium halides used in the invention examples thereof show that the term halide in this patent is that given in the "McGraw-Hill Dictionary of Scientific and Technical Terms", Ed. S. P. Parker, McGraw-Hill Book Company, New York (1989) i.e. "a compound of the type MX, where X is fluorine, chlorine, bromine or astatine, and M is another element or organic radical". Onium halides are, however, not able to stabilize fully photothermographic materials against fogging.
U.S. Pat. No. 3,957,517 discloses a dry method for the stabization of a print-out silver halide photographic material wherein the silver halide is the image-forming substance, which comprises (1) imagewise exposure of a silver halide photographic emulsion material to form a print-out image, wherein said silver halide photographic emulsion material consists essentially of silver halide grains at least 50 mol % of which consists of silver bromide and 0.1 to 33 mol %, based on the silver halide, of an onium compound having an iodide ion or an anion containing iodine, said onium compound being a member selected from the group consisting of compounds of the formulae: ##STR1##
wherein A is N, P, As or Sb, B is O, S, SO, Se or Sn, and each of R.sub.1 to R.sub.9 is an alkyl group having less than 8 carbon atoms, or an aryl group having less than 16 carbon atoms, or of said R.sub.1 to R.sub.9 two groups can be connected to each other to form a ring selected from the group consisting of a pyridine ring, a morpholine ring, an oxazine ring, a thiazine ring, a thiazole ring, an oxazole ring, a benzothiazole ring and a benzo-oxazole ring, and wherein X is an iodine ion or an iodine containing anion; and subsequently heating said photographic material to a temperature of at least 80.degree. C., thereby stabilizing said print-out image.
In U.S. Pat. No. 4,173,482 a dry image forming material is disclosed capable of forming an image by preliminary heating, imagewise exposure to light, and heat development thereof, said material comprising (a) a non-photosensitive organic silver salt oxidizing agent, (b) a reducing agent for a silver ion and (c) at least one halogen molecule selected from the group consisting of a bromine molecule, an iodine molecule, iodine chlorides, iodine bromide and bromine chloride and optionally further comprising at least one halogen ion source in addition to said at least one halogen molecule. According to U.S. Pat. No. 4,173,482 (column 3, lines 64-68) the halogen ion source and the halogen molecule may be added separately, or a compound or compounds capable of forming a halogen ion source and a halogen molecule by reaction at the preparation of the image forming material may be used, for example triphenylphosphite nonaiodide. The use of single compound, acting as a halogen ion source and providing a halogen molecule requires the use of an additional preliminary heating step prior to image-wise exposure to actinic light and also partially converts in-situ the non-photosensitive organic silver salt oxidizing agent present into silver halide which is undesirable as regards control over the gradation of the image as expressed by the dependence of optical density upon exposure to actinic light.
U.S. Pat. No. 5,028,523 discloses a photothermographic emulsion comprising silver halide, light-insensitive silver oxidizing compound, reducing agent for silver ion, and a binder, said emulsion also comprising a hydrobromic salt of a nitrogen-containing heterocyclic ring compound associated with a pair of bromine atoms. However, the hydrobromic salt of a nitrogen-containing heterocyclic ring compound associated with a pair of bromine atoms used in the invention example of U.S. Pat. No. 5,028,523, pyridinium hydrobromide perbromide, exhibits a high reactivity with silver behenate to form photosensitive silver bromide which, as is mentioned above, is undesirable as regards control over the gradation of the image as expressed by the dependence of optical density upon exposure to actinic light.